The colour imparted by Co(II) compounds to glass is

Glass is an amorphous and transparent or translucent solid obtained by solidification of a mixture of silicates of differentmetals, one of which is always an alkali metal. Addition of small amounts of transition metal salts to the glass mixture imparts colour to the glass. For example, addition of small amounts of $\mathrm{Fe}^{3+}, \mathrm{Fe}^{2+}, \mathrm{Co}^{2+}, \mathrm{Cr}^{3+}$ and $\mathrm{Mn}^{4+}$ salts imparts yellow-brown, light blue, deep blue, green or violet colour respectively.